1. Field of the Invention
Our invention relates to tensile testing systems and is directed more specifically to a system for opto-electronically testing the stress-strain properties of materials under tension. The materials suitable to be tested by the system of our invention includes, for example, rubbers, fibers, and plastics.
2. Description of the Prior Art
In the tensile testing of the above listed materials, the ends of a specimen tend to slip away from the pair of chucks or gripping means of the testing machine as the tensile stress on the specimen increases. The distance between the chucks, therefore, does not provide the true measure of the specimen elongation. The usual practice heretofore adopted to overcome this problem has been to mark the specimen with a pair of longitudinally spaced lines and to visually measure the spacing therebetween with the aid of a scale.
Such visual or manual measurements, however, have poor reproducibility and are subject to significant variation from individual to individual. Furthermore, where the tensile moduli (i.e., the quotient of the tensile stress divided by the cross sectional area of the specimen) at, for example, 100, 200 and 300 % specimen elongations are required, these elongations must be ascertained visually even though provision may be made for automatic measurement of the stresses on the specimen. The manual measurement of specimen elongation has thus been highly troublesome, time-consuming, and has required an undue amount of manpower.
We are aware of several instruments already suggested for automatic measurement of specimen elongation, but these prior art instruments are unsatisfactory for one or other of the following reasons. In some instances some special appendages must be attached to specimens, such appendages including armatures for detection of magnetism or levers for actuation of differential transformers. According to another known system wherein photoelectric elements are employed for detecting and tracking the markings on a specimen, detector means must be disposed in the close proximity of the specimen, and a drive mechanism is also required for tracking the specimen markings.
In both cases considerable difficulties are encountered as to the mounting and demounting of specimens on and from the testing machine. With these prior art systems, moreover, positional adjustment of the armatures and the photoelectric elements must be made in preparation for actual testing.